[Bruce Lancaster]

Maybe so! I was seeing that central hole as a trapezoid...on looking now, it could well be round with some visual distortion from crud and grainy image!
Slap a ruler on there quick...I suspect Z is correct and you have a 99!
On the costs...forget externals like heads and manifold if you are broke. That stuff can be added later when you recover from first steps or hit lucky in your scrounging.
Bore and valve work is about the same as anything, if you scrounge and swap sleeves yourself cheaper. Pistons...poor people don't use Ross, and Joblot traditionally has deals on the standard pistons you can use if you replace sleeves...
Bearings and such cost more than SBC, rods may turn out ready to clean and re-use or turn out to be another expense...carefully split good rebuild versus parts that can be swapped later, think about the Merc crank, a somewhat pricey beneficial change that you can't really do later...
Now MEASURE THAT BORE WITH A RULER! I am thinking Zaemo saw that middle hole more clearly than I did and you have a 99, a big upgrade!